From the beginning of powered aviation, aircraft have been under certain flying conditions troubled by accumulations of ice on component surfaces of aircraft such as wings and struts. If unchecked, such accumulations can eventually so laden the aircraft with additional weight and so alter the airfoil configuration of the wings and control surfaces of that aircraft so as to precipitate an unflyable condition. Efforts to prevent and/or remove such accumulations of ice under flying conditions has resulted in three generally universal approaches to removal of accumulated ice, a process known generally as de-icing.
In one form of de-icing, known as thermal de-icing, leading edges are heated to loosen adhesive forces between accumulating ice and the aircraft component. "Leading edges" as used herein means those edges of an aircraft component on which ice accretes and are impinged upon by air flowing over the aircraft and having a point or line at which this airflow stagnates. Once loosened, this ice is generally blown from he aircraft component by the airstream passing over the aircraft.
There are two popular methods of heating leading edges. In one approach known as electrothermal de-icing, an electrical heating element is placed in the leading edge zone of the aircraft component, either by inclusion in a elastomeric boot applied over the leading edge, or by incorporation into the skin structure of the aircraft component. This heating element is typically powered by electrical energy derived from a generating source driven by one or more of the aircraft engines and is switched on and off to provide heat sufficient to loosen accumulating ice. In small aircraft, a sufficient quantity of electrical power may be unavailable for use of electrothermal de-icing.
In the other heating approach, gases at elevated temperature from one or more compression stages of a turbine engine are circulated through the leading edges of components such as wings and struts in order to cause a de-icing or anti-icing effect. This approach is employed typically only in aircraft powered by turbine engines by draining off compressed air having an elevated temperature from one or more compressor stations of a turbine engine. This approach can result in reduced fuel economy and lower turbine power output.
The second commonly employed method for de-icing involves chemicals. In limited situations, a chemical has been applied to all or part of an aircraft to depress adhesion forces associated with ice accumulation upon the aircraft or to depress the freezing point of water collecting upon surfaces of the aircraft.
The remaining commonly employed method for de-icing is typically termed mechanical de-icing. In the principal commercial mechanical de-icing means, pneumatic de-icing, the leading edge zone or wing or strut component of an aircraft is covered with a plurality of expandable, generally tube-like structures that are inflatable employing a pressurized fluid, typically air. Upon inflation, the tubular structures tend to expand substantially the leading edge profile of the wing or strut and crack ice accumulating thereon for dispersal into the airstream passing over the aircraft component. Typically, these tube-like structures have been configured to extend substantially parallel to the leading edge of the aircraft. These conventional low pressure pneumatic de-icers are formed from compounds having rubbery or substantially elastic properties. Typically, the material forming the inflatable tubes on such de-icer structures can expand or stretch by 40% or more during an inflatable cycle, thereby causing a substantial change in the profile the de-icer as well as in the leading edge to thereby crack ice accumulating on the leading edge.
These conventional pneumatic de-icers require a large volume of air to inflate their highly expandable tubes and the time for inflating such tubes typically and historically has averaged from about two and six seconds. The distortion of the airfoil profile caused by inflation of the tubes can substantially alter the airflow pattern over the airfoil and adversely affect the lift characteristics of the airfoil. The rubber or rubber-like materials forming these conventional pneumatic de-icers typically are possessed of a Young's modulus (modulus of elasticity) of approximately 6900 kPa. The modulus of elasticity of ice is variously reported as being between about 275,000 kPa and about 3,450,000 kPa. Ice is known to be possessed of an elastic modulus enabling typical ice accumulations to adjust to minor changes in contours of surfaces supporting such ice accumulations. The modulus of elasticity of rubber compounds used in conventional de-icers is much lower than the modulus of elasticity typically associated with ice accumulations. The large expansion of conventional pneumatic de-icers has functioned to crack or rupture the structure of the ice accumulations thereby allowing such accumulations to be swept away by impinging windstreams.
Other mechanical means for effecting de-icing include electromechanical hammering such as that described in U.S. Pat. No. 3,549,964 to Levin et al. Concern respecting the susceptibility of the airfoil skin to stress fatigue upon being hammered over extended periods of time have functioned in part to preclude substantial commercial development or adoption of such technique.
Another electromechanical ice removal system is described in U.S. Pat. No. 4,690,353 to Haslim et al. One or more overlapped flexible ribbon conductors, each of which is folded back on itself, is embedded in an elastomeric material. When a large current pulse is fed to the conductor, the anti-parallel currents in the opposed segments of adjacent layers of the conductor result in interacting magnetic fields producing an electrorepulsive force between the overlying conductor segments causing them to be separated near instantaneously. This distention tends to remove any solid body on the surface of the elastomeric material.
Another electromechanical ice removal system is described in U.S. Pat. No. 4,875,644 to Adams et. al., the teachings of which are herein incorporated by reference. Two or more sheet-like arrays, each containing in spaced apart relationship a plurality of parallel ribbon-shaped electrical conductive members, are rapidly and forcefully driven apart when a large magnitude current pulse is fed to the conductors.
U.S. Pat. Nos. 4,706,911 to Briscoe et al. and 4,747,575 to Putt et al. disclose apparatus for de-icing leading edges in which an impulse of fluid under pressure is utilized to rapidly inflate an inflation tube positioned between a support surface and a sheet-like skin possessed of a substantially elevated modulus. The impulse of fluid is delivered to the inflation tube causing the high modulus skin to dislocate and then stop suddenly. Momentum imparted to the ice accumulations thereby causes additional ice movement which assists in ice detachment and dislodgement. The inflatable tubular structure in certain preferred embodiments is inflated within not more than about 0.1 second and preferably not more than about 0.5 milliseconds. FIG. 4 and the attendant description of U.S. Pat. No. 4,706,911 describe an ejector/pilot operated discharge valve suitable for use in such pneumatic impulse de-icers. In FIG. 7 and the attendant description of U.S. Pat. No. 4,747,575 there is described a chattering valve for use in a pneumatic impulse de-icer which delivers a rapid series of fluid pressure pulses to the inflatable tube of a de-icer apparatus affixed to a leading edge. Efforts to improve such pneumatic impulse de-icing systems have led to continuing efforts to improve valves for delivery of the desired fluid impulse.
While the devices and methods disclosed in the foregoing patents have been found to be suitable for de-icing of aircraft, it remains a desired goal of the industry to reduce weight and increase service life and reliability wherever possible. Toward these objectives modern aircraft designers and manufacturers are specifying with increasing frequency use of lightweight composite materials manufactured from high modulus fibers including, but not limited to, carbon, graphite, aramid, and glass in matrices of organic resins or carbon. Leading edge surfaces such as those found on wings and struts of aircraft and tail sections have been provided with separately manufactured apparatus such as that disclosed in U.S. Pat. Nos. 4,706,911 and 4,747,575. Such apparatus have been fitted to existing wing structures by adhesive bonding of such auxiliary de-icing apparatus. Such auxiliary devices change the contour of the leading edge by virtue of their presence, an undesired consequence. As an alternative, at the time of design or prior to fitting of such an s apparatus, the leading edge of the airfoil of certain prior art embodiments has been modified so as to provide a recess for fitment of the de-icing apparatus. This latter manner of providing de-icing apparatus resulted in a finished assembly having smooth airflow characteristics due to the provision of such recess. However, in this instance, the underlying support surface and airfoil has been structurally complete without addition of the de-icing apparatus. The de-icing apparatus provided no additional or minimal additional structural reinforcement to the underlying support surface to which it was affixed. Many of the heretofore known accessory de-icing apparatuses were provided with an outer ice accreting surface formed of elastomeric material such as rubber (neoprene) or urethane. These materials are far more susceptible to erosion from the action of rain, sleet, hail, and snow during flight than the conventional aluminum alloy leading edge surface employed on modern large commercial and certain general aviation and commuter aircraft. Such aircraft have a service life expectancy of twenty or more years, including the aluminum alloy skin which is typically from about 0.025 inch to about 0.190 inch thick. It is an objective of the aircraft industry to minimize repair or replacement of such de-icing apparatus. Ideally, it is desired that the de-icing apparatus include an ice accreting surface having a resistance to rain erosion at least equal to that of the aluminum alloy skin currently employed on large turbine powered commercial aircraft which are de-iced using bleed air systems.
Rain is not the only type of impact that leading edges encounter. Impacts by birds, hail, and debris kicked up from the runway and accidents during routine aircraft maintenance are also likely during the operational life of the aircraft.
In addition to the various improvements needed for airfoil and de-icing mechanisms, known pneumatic de-icers can also exhibit high stress points and bending moments typically near the anchor points of the inflatable tubes. Adjacent anchor points result in a "dead space" or inactive area where, despite actuation or inflation of either tube, no displacement occurs resulting in a high stress concentration. These inactive areas of high stress can cause fatigue in the outer skin particularly for high tensile modulus outer skins made of metal.